How to edit xyz F-Curves at once?
The key to the default behavior is in how you setup the first two keyframes.

Let's say that in the first keyframe you have x/y/z scale at 1.000

Then in the second keyframe you have x/y/z scale at 2.000

Then naturally your curves geometrically follow the same rate of change.
Blender somehow decides to lock this on you though, and when you try to edit one axis in the Graph editor the other two follow.

If this is not desired you can always make a slight change in the 3D view's  >> Properties Region >> Transform >> Then x/y/z scale. If you don't see this, with your mouse cursor over the 3D view hit N to toggle.

If you have a differing starting scenario (your axis values aren't all the same), you can still achieve similar behavior by using the Normalize toggle within the Graph Editor.



This will visually group the graph curves together for you to edit them as one curve.

This however will fix your offsets from one axis channel to the other, and may not be the desired effect depending on what you are trying to accomplish.

To solve that you can always use your 2D cursor options to control where to base action from.